First of all Scott Kelly, who was the astronaut who actually spent the near year in space comes off as compelling and interesting, and the contrast with his twin brother is not so much about the effects of that -- but just how relatively narcissistic Mark is.
In terms of the experience and science Scott is supposed to be the subject, Mark Kelly is simply the control, yet Mark effectively "photobombs" throughout, like he is using this series as a launchpad for his political aspirations. It doesn't belong.
Moreover, and perhaps as part of that phenomena, as others have noted, the actual science, the data, is missing from this series.